1. Field of the Invention
The present invention generally relates to a fixing device fixing an unfixed image onto a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
An image forming apparatus such as a copier, a facsimile machine, a printer, and a printing machine has been used to obtain a copy or recorded medium by fixing (i.e., heating and pressing) an unfixed image onto a recording medium, the unfixed image having been transferred and carried on the recording medium. In the fixing, the recording medium on which the unfixed image is carried is heated and pressed while being fed and sandwiched, so that a developer, especially toner, included in the unfixed image is melted and softened to penetrate into the recording medium. By doing this, the toner may be fixed into the recording medium.
In such a image forming apparatus, in order to reduce a warm-up time and reduce energy consumption, there is a demand for improving accuracy of temperature control. To that end, there is a demand for improving detection accuracy of internal temperature sensors. As a temperature sensor to detect the temperature of a fixing roller (of a fixing device), a thermopile is generally used due to higher accuracy. On the other hand, as a temperature sensor to detect the temperature of a pressing roller (of the fixing device), a non-contact-type thermistor having lower detection accuracy than that of the thermopile may be used. There are various methods of performing temperature control in an image forming apparatus. Some of the methods are briefly described below.
Japanese Patent No. 3478761 discloses a technique in which a non-contact-type temperature sensor includes a thermopile and a thermistor to detect the temperature of the non-contact-type temperature sensor. Then, a temperature increase of the thermistor and a temperature increase of the thermopile are compared. Based on the comparison result, a degree of contamination on the surface of the non-contact-type temperature sensor is detected and a temperature compensation is performed.
Further, Japanese Patent No. 2968054 discloses a technique in which there is a switching unit that changes the position of a thermistor between a contacting position where the thermistor is in contact with the fixing roller and a non-contacting position where the thermistor is not in contact with the fixing roller. The thermistor is in contact with the fixing roller during a warm-up time and a standby more where the fixing roller does not rotate and measures the temperature of the fixing roller. On the other hand, during the recording medium is fed (i.e., during the fixing roller rotates), the thermistor detects the temperature of the fixing roller while being separated from the fixing roller. By operating the thermistor in this way, it becomes possible to prevent the damage of the surface of the fixing roller by the thermistor.
Further, Japanese Patent No. 3777722 discloses a technique in which a non-contact type first temperature sensor is disposed at a position corresponding to a sheet feeding region of the fixing roller, and a contact-type second temperature sensor is disposed at a position corresponding to a non-sheet feeding region of the fixing roller. Whether the warm-up operation is finished is determined by using the second temperatures sensor corresponding to the non-sheet feeding region. On the other hand, while a recording medium is being fed, the first temperature sensor corresponding to the sheet feeding region is used for temperature control. During the warm-up operation, the non-contact type first temperature sensor is used. Therefore, it becomes possible to eliminate an influence due to the contamination of the surface of the non-contact type first temperature sensor. As a result, unnecessary extension of the warm-up time may be prevented.
Further, Japanese Patent Application Publication No. 2000-194228 discloses a technique in which plural sensors having different temperature characteristics from each other are disposed close to the fixing roller, so that an appropriate sensor is selected depending on the use temperature. As a result, it becomes possible to accurately detect a temperature in any of the temperatures ranges.